Polyurethanes of differing types are produced by the polymerization of diisocyanates, for example 4,4′-methylenebis(phenyl isocyanate), MDI for short, or 2,4-tolylene diisocyanate, TDI for short, with polyether polyols or polyester polyols. Polyether polyols are obtainable for example by alkoxylation of polyhydroxy-functional starters. Examples of common starters are glycols, glycerol, trimethylolpropane, pentaerythritol, sorbitol or sucrose. Polyurethane foams are produced using additional blowing agents, for example pentane, methylene chloride, acetone or carbon dioxide. It is customary to use surface-active substances, especially surfactants, to stabilize the polyurethane foam. Apart from a few purely organic surfactants, usually silicone surfactants are used because of their higher interface stabilization potential.
A multiplicity of different polyurethane foams are known, examples being hot-cure flexible foam, cold-cure foam, ester foam, rigid PUR foam and rigid PIR foam. The stabilizers used here have been specifically developed to match the particular end use, and typically give a distinctly altered performance if used in the production of other types of foam.
In the prior art, the polysiloxane-polyoxyalkylene block copolymers used for polyurethane foam stabilization are generally produced by noble metal-catalyzed hydrosilylation of unsaturated polyoxyalkylenes with SiH-functional siloxanes, so-called hydrogen siloxanes, as described in EP 1 520 870 for example. The hydrosilylation can be carried out batchwise or continuously, as described for example in DE 198 59 759 C1.
A multiplicity of further documents, such as EP 0 493 836 A1, U.S. Pat. No. 5,565,194 or EP 1 350 804 for example, each disclose specifically assembled polysiloxane-polyoxyalkylene block copolymers to achieve specific performance profiles for foam stabilizers in diverse polyurethane foam formulations. The respective stabilizers are frequently prepared using, in the hydrosilylation, mixtures of two or three preferably endcapped allyl polyethers whose molecular weights are less than 6000 g/mol and preferably less than 5500 g/mol. Polyethers having molecular weights above 5500 g/mol are not readily obtainable via alkaline alkoxylation, since competing reactions favoring chain termination come to dominate with increasing chain lengths.
In view of the fact that the availability of fossil resources, namely mineral oil, coal and gas, is limited in the long run and against the background of rising crude oil prices, there has been increased interest in recent years in using polyols based on renewable raw materials for producing polyurethane foams (WO 2005/033167 A2; US 2006/0293400 A1). In the meantime, a whole series of these polyols has become available on the market from various producers (WO2004/020497, US2006/0229375, WO2009/058367). Depending on the source of the raw material (e.g., soybean oil, palm oil or castor oil) and the subsequent processing steps, the polyols obtained differ in their property profiles. Essentially two groups can be distinguished:
a) polyols based on renewable raw materials which are modified such that they can be used at 100% for production of polyurethane foams (WO2004/020497, US2006/0229375),
b) polyols based on renewable raw materials which, owing to their processing and properties, can replace the petrochemically based polyol to a certain extent only (WO2009/058367, U.S. Pat. No. 6,433,121).
Especially the use of vegetable polyols of group B has distinct repercussions for the production of flexible polyurethane block foams, both for the process management and the physico-chemical properties of the resulting foam. For instance, the use of vegetable polyols produced from soybean oil or palm oil leads with increasing use level, under otherwise unchanged processing conditions, to a lengthening in rise time, a change in hardness and air permeability and also to reduced elongation at break, tensile strength and elasticity for the foam. Some changes, for example rise time and air permeability, can be held in check by appropriately adapting the formulation, i.e., for example the catalyst combination. Other physical properties such as, for example, hardness, elongation at break, tensile strength and elasticity remain adversely changed, however.
The problem addressed by the present invention is therefore that of providing stabilizers/additives that make it possible to improve the physical properties of flexible polyurethane slabstock foams comprising a high proportion of polyols having a vegetable origin.